This project deals with phenomena characterizing a house mouse population as it proceeds through successive stages of crowding in an octagonal, designed habitat. The 8 pie-shaped cells of the habitat are subdivided into 2 subcells with the nest boxes in the counter-clockwise subcell about one-half the distance from the resources as are those in the clockwise subcell. The growth of the population has been controlled, such that after the adult population reached 100, the population has been allowed to double every 210 days and now stands at 1600 adults. With increasing density there have been several changes in social structure and in space utilization. At lower densities more animals nested in the counter-clockwise subcells than in the clockwise subcells, while at high density there are about equal numbers of mice nesting in each subcell. Within the habitat, in 4 adjacent cells there are (1) more adults, (2) many more adults nesting in public space, and (3) higher infant mortality than in the other 4 adjacent cells. These differences are suggestive of a developing behavioral sink in that half of the habitat with the higher density.